I think you have to get a dolmus from Sirince to Selcuk, then the mini bus to Kusadasi, then the mini bus to Soke to get the Didim Seyhat back to Didim, sorry don't know the times but the last Didim Seyhat back to Didim runs about 8.30pm, Or you could go on one the day trips from one of the tour companies in Didim